BRACHIOPODS. Ill 



valve mollusca. Like them, they are enclosed in a bivalve 

 shell ; but this is always fixed or cemented for life to one 

 spot. They were crowded beyond calculation in former seas 

 before the present race of marine mollusca had existence, 

 but few have survived the cataclysm, and, from their present 

 rarity, they are highly prized by collectors. Cuvier reduces 

 them all to three genera ; the Lingula, a sort of small mussel 

 hanging from rocks in deep water by a cylindrical fleshy 

 pedicle ; the Terebratula, something like a Pecten or Cockle, 

 with the under valve perforated to give passage to the pedicle 

 by which the shell is attached ; and the Orbicula, known by 

 its circular figure and the inequality of the valves, the upper 

 being elevated into a shallow cone, while the lower is flat 

 and cemented to the rocks. 



This letter, however dry and tedious in its details, will 

 require more than one perusal to fix the names of the prin- 

 cipal tribes on your memory ; for unless these are familiar 

 as household words our progress must be halting : 



" names are good ; for how, without their aid, 



Is knowledge gained by man, to man conveyed ? " 



